Although bone screws are known, the threads on bone screws have been given scant attention. The threads anchor the screw into the bone. The treads keep the screw from being axially pulled out of the bone. The threads cut a helical path into the bone as the screw rotates into the bone. Given the importance of the screw threads, it is surprising that there was no reference known to me on the design of screw thread shapes.
There are many variable aspects of thread shape. The thread can have a sharp or blunt apex. The pitch of the thread can be varied. A shallow thread pitch provides many turns around the screw with each turn close to adjacent turns. A steep pitch provides few turns with the turns spaced far apart. The height of the thread from the surface of the screw core to the apex of the thread can be deep or shallow. The thickness of the thread can be varied.
Threads have a superior surface on the side facing the screw head and an inferior surface facing the screw tip. The engagement of the superior surface with the bone provides resistance to screw pull-out. The angle between the inferior and superior surfaces is the thread angle. If this angle is small, the thread is narrow as is a knife. If the thread angle is large, then the thread is wide and strong. The need for a knife-like thread to cut through the bone must be balanced against the need for a strong thread.
There are a few references regarding thread shape. U.S. Pat. No. 4,463,753, issued Aug. 7, 1984, entitled "Compression Bone Screw" discloses that the angle made by a thread cross-section is "critical" to the operation of the screw. The angle between the superior and inferior thread surface should be between 30 and 50 degrees, with 40 degrees being optimal according to the '753 Patent. Threads having an angle of less than 20 degrees are weak and can fail. If the angle is greater than 60 degrees, the tread will strip out the bone between the treads. Similarly, U.S. Pat. No. 4,870,957, issued Oct. 3, 1989, and entitled "Ligament Anchor System" discloses a range of angles for the inferior and superior surfaces of the threaded studs that it discloses.
I have found that bone screws can be improved by varying the cross-sectional shape of the thread from the tip to the head of a screw. The thickness of the thread near the tip of the screw has a narrow cross-section. The narrow thread easily cuts into the bone as the screw rotates into the bone. There is minimal tearing and displacement of the bone by the insertion of the narrow thread. The thread becomes thicker toward the head of the screw to increase thread strength and to displace bone matter downward against the superior thread surface.
The cross-section of the thread gradually thickens from the tip to the head of the screw. It is preferable that the thread be thickened by increasing the angle between the inferior thread surface and a line normal to the screw axis. This angle is smallest at the screw tip and widest at the head. The angle of the superior surface is constant along the length of the screw. By thickening the thread, the inferior surface shifts downward toward the adjacent superior surface. Accordingly, as the thread rotates into the bone, the inferior thread surface gradually displaces the bone matter between the threads down against the adjacent superior thread surface.
The bone matter is compressed against the superior thread surface and partially rotated downward against the superior surface. Compressing bone matter against the superior surface increases the bone resistance to thread pull out. Similarly, rotating bone matter downward aligns the bone matter to enhance the resistance to axial load forces on the screw.
By increasing the thread thickness in the direction of the screw head, the thread is thickest and strongest near the head. This is particularly advantageous in bones because of the hard cortical bone shell. The cortical bone is harder and more compact than the spongy cancellous matter in the center of bones. The cortical bone provides the bulk of the bone's resistance to screw pull-out forces (axial load forces). The thread near the screw head engages the cortical bone and, thus, carries much of the axial load on the screw. The thicker threads of the present invention are optimal for supporting large loads at the cortical bone. Moreover, the varying thickness of the thread compresses and rotates the cortical bone downward against the superior screw surface to provide enhanced pull-out resistance. Also, the thicker core at the head enhances the lateral load to failure strength of the screw. The core is thickest near the head which is where the greatest lateral compression and distractive forces act on the screw. These forces act to bend the screw Just below the head and the thick core near the head provides the greatest resistance to bending.
It is an object of my invention to provide an improved screw for bones, laminated woods, and other materials. In particular, it is an object of my invention to enhance the pull out resistance of screws for bones and other materials. In addition, it is an object of my invention to provide a screw able to withstand large load forces without shearing off the head of the screw or pulling the screw out of its position.